The invention relates to a hydraulic system for a working machine. The hydraulic system is a load sensing (LS) system and comprises a hydraulic actuator for movement of an implement and a control valve having an inlet valve and an outlet valve for controlling the flow of hydraulic fluid from a pump to the hydraulic actuator, and for draining hydraulic fluid from the hydraulic actuator. The system also comprises a means for determining the load on the hydraulic actuator. The invention also relates to a method and a control unit for controlling a hydraulic system.
The invention can be applied on different types of hydraulic system, for example hydraulic systems for operating hydraulic cylinders for lifting an arm or tilting an implement of a wheel loader or for operating hydraulic cylinders for a dump body of an articulated hauler.
Although the invention will be described with respect to a hydraulic system for a wheel loader, the application of the invention is not restricted to this particular application, but may also be used in other hydraulic systems and vehicles.
A working machine is usually provided with a bucket, container or other type of implement for digging, lifting, carrying and/or transporting a load.
For example, a wheel loader has a lift arm unit for raising and lowering an implement, such as a bucket. The lift arm unit comprises hydraulic cylinders for movement of a load arm and the implement attached to the load arm. Usually a pair of hydraulic cylinders is arranged for raising the load arm and a further hydraulic cylinder is arranged for tilting the implement relative to the load arm.
In addition, the working machine is often articulated frame-steered and has a pair of hydraulic cylinders for turning/steering the working machine by pivoting a front section and a rear section of the working machine relative to each other.
The hydraulic system generally further comprises at least one hydraulic pump, which is arranged to supply hydraulic power, i.e. hydraulic flow and/or hydraulic pressure, to the hydraulic cylinders. The hydraulic pump is driven by a power source, such as an internal combustion engine or an electric motor. The hydraulic system of a working machine is usually a so called load sensing system (LS-system). This means that the pump that provides the actuators with hydraulic fluid receives a signal representing the current load pressure of a hydraulic cylinder in operation. The pump is then controlled to provide a pressure which is somewhat higher than the load pressure of the hydraulic cylinder.
The hydraulic pump is often a variable displacement pump that is driven by the prime mover of the working machine. If the pump is driven by an internal combustion engine, the pump is connected to a power take-off which can be located between the internal combustion engine and a transmission arrangement, such as a gear box. The transmission arrangement is in turn connected to e.g. wheels of the working machine for the propulsion thereof.
When driving a hydraulic cylinder in an LS-system, hydraulic oil is supplied by a pump and the flow of hydraulic oil from the pump is directed by an inlet valve to one side of the hydraulic cylinder and the flow of hydraulic oil from the other side of the hydraulic cylinder is drained to tank by an outlet valve.
The inlet valve and the outlet valve can be integrated in the same spool of a control valve. This means that when the valve is controlled to connect the pump to the piston rod side of the hydraulic cylinder, the piston side of the hydraulic cylinder is connected to tank, and when the pump is connected to the piston side of the hydraulic cylinder, the piston rod side of the hydraulic cylinder is connected to tank. This gives a robust system and relatively low costs.
A disadvantage with such a system is however that the pump is always supplying hydraulic oil to the hydraulic cylinder also during operations where there is no need for the pump to drive the hydraulic cylinder. For example when lowering a load, the mass of the load would often be sufficient to achieve the lowering movement without any pressure generated by the pump. This in turn means that during certain operations energy losses (increased fuel consumption) occur due to the use of the hydraulic pump even if no pump work is needed by the hydraulic cylinders.
An object of the invention is to provide a hydraulic system, by which system the energy losses and thereby the fuel consumption can be reduced.
The invention is based on the insight that by the provision of a hydraulic system that comprises a valve for disconnecting the flow of hydraulic fluid from the pump to the hydraulic actuator, while allowing another flow of hydraulic fluid to the hydraulic actuator, provided that a determined load on the hydraulic actuator exceeds a threshold value, the energy losses can be reduced due to the fact that the pump must not be driven when no pump work is required or the pump can be used for another hydraulic function.
For example, during lowering of a load when the pump is connected to the piston rod side of a hydraulic cylinder by means of the inlet valve, and the piston side of the hydraulic cylinder is drained by means of the outlet valve, the pump can be cut off by means of the disconnecting valve, while hydraulic fluid required for filling the piston rod side of the hydraulic cylinder is available from the return line and/or tank, and the piston rod of the hydraulic cylinder can be moved due to the mass of the load on the actuator. The load on the actuator can be caused by the actual mass of the load (in a bucket) to be lowered and/or the dead load (mass of bucket and/or lift arm).
According to one embodiment of the invention, the hydraulic system comprises a load holding valve arranged downstream the control valve and upstream the actuator with respect to the flow direction from the pump to the hydraulic actuator, and said valve for disconnecting the flow of hydraulic fluid from the pump to the hydraulic actuator is arranged to provide a pilot pressure to the load holding valve, thereby closing the load holding valve and disconnecting the pump. Hereby a disconnection valve working with relatively low flow of hydraulic fluid can be used for controlling the load holding valve working with a relatively high flow of hydraulic fluid to the actuator. Since such a load holding valve is very often used in this type of hydraulic system, no additional full flow valve has to be added.
According to a further embodiment of the invention, the system comprises a valve for preventing an LS-signal based on the load on the hydraulic actuator from reaching the pump when the pump is disconnected. Hereby, when the flow of hydraulic fluid from the pump to the hydraulic actuator is disconnected, the pump can be controlled to provide a lower (stand by) pump pressure by changing the displacement of the pump. Alternatively, the pump may receive an LS-signal from another actuator for supplying the pressure required for this actuator.
Preferably, the valve for preventing an LS-signal based on the load on the hydraulic actuator from reaching the pump when the pump is disconnected and said valve for disconnecting the flow of hydraulic fluid from the pump to the hydraulic actuator provided that a determined load on the hydraulic actuator exceeds a threshold value, is one and the same valve. Hereby, a cost effective system where the pump can be disconnected at the same time as the LS-signal is prevented from reaching the pump can be achieved.
According to a further embodiment of the invention, the load determining means comprises a pressure sensor arranged for measuring a hydraulic pressure indicating the load pressure of the hydraulic actuator. Hereby, the load on the hydraulic actuator can be determined and compared to the threshold value for deciding whether or not the flow of hydraulic fluid from the pump to the hydraulic actuator is to be disconnected.
According to a further embodiment of the invention, the threshold value for the load on the hydraulic actuator is calculated based on a signal indicating requested velocity of the hydraulic actuator. Hereby, the disconnection of the pump is not only dependent on the actual load on the actuator, but also on the requested velocity, preferably such that for a relatively low requested velocity of the actuator the load threshold value is lower than the load threshold value for a relatively high requested velocity of the actuator.
According to a further aspect, the invention relates to a method for controlling a hydraulic system.
According to a further aspect, the invention relates to a control unit for controlling a hydraulic system.
The same advantages as discussed above with reference to the hydraulic system can be reached by the method and the control unit according to the invention. Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.